Many subterranean waters contain alkaline earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulfate, bicarbonate, carbonate, phosphate, and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when the concentrations of the barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. Solubility products are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
Barium and strontium sulfate scale deposits present a unique and particularly intractable problem. Under most conditions, these sulfates are considerably less soluble in all solvents than any of the other commonly encountered scale-forming compounds, as shown by the comparative solubilities given in Table 1 below.
TABLE 1 ______________________________________ Comparative Solubilities, 25.degree. C. in Water. Scale Solubility, mg./l. ______________________________________ Gypsum 2080.0 Strontium sulfate 140.0 Calcium Carbonate 14.0 Barium sulfate 2.3 ______________________________________
It is generally acknowledged that barium sulfate scale is extremely difficult to remove chemically, especially within reasonably short periods of time: the solvents which have been found to work generally take a long time to reach an equilibrium concentration of dissolved barium sulfate, which itself is usually of a relatively low order. Consequently, barium sulfate must be removed mechanically or the equipment, e.g. pipes, etc., containing the deposit must be discarded.
The incidence of barium sulfate scale is worldwide, and it occurs principally in systems handling subsurface waters. Because of this, the barium sulfate scale problem is of particular concern to the petroleum industry as water is generally produced with petroleum and as time goes on, more petroleum is produced by the waterflooding method of secondary recovery, implying even greater volumes of produced water. The scale may occur in many different places, including production tubing, well bore perforations, the area near the well bore, gathering lines, meters, valves and in other production equipment. Barium sulfate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well.
Barium sulfate scale is particularly troublesome when sulphate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. This particular aspect of the barium scale problem is severe in some U.S. oil fields as well as some older North Sea oil fields. Scaling of this nature is also expected to occur during advanced production stages in other North Sea fields particularly after seawater breakthrough has taken place.
Another problem associated with the formation of barium and strontium sulfate scales is that radium, another member of the alkaline earth group of metals, tends to be deposited at the same time so that the equipment becomes radioactive, and may eventually have to become unusable for safety reasons alone. At present, a considerable amount of oilfield tubular goods are in this condition and cannot be readily restored to usable condition because of the difficulty of removing the radioactive scale.
As noted in Ser. No. 07/369,897, various proposals have been made in the past for the removal of barium sulfate scales using chemical scale removal compositions. Examples of such scale removal techniques are to be found in U.S. Pat. Nos. 2,877,848 (Case), 3,660,287 (Quattrini), 4,708,805 (D'Muhala) and U.S. Pat. Nos. 4,190,462; 4,215,000 and 4,288,333 (DeJong). The proposals set out in these dislosures represent, however, only partial or unsatisfactory solutions to the scale removal problem.
The related applications identified above disclose highly effective chemical scale removal compositions which may be used for removing alkaline earth sulfate scales from oil field equipment including tubular goods such as pipe, casing, and wellhead equipment such as meters, valves and above-ground piping. Reference is made to these copending applications for disclosures of these scale removal compositions and the methods by which they may be used.
Although, as described here, a principal problem area is with the deposition of these scale materials on oilfield equipment, there is a related problem which is that the earth around the equipment may become contaminated with these scale materials together with any associated radioactive components, if the subsurface waters are allowed to spill out onto the surface of the earth during normal operations. This may occur, for example, if subsurface waters are brought to the surface during drilling or production and stored in ponds from which they can seep into the surrounding earth layers. Another source of contamination of this kind is when attempts are made to remove scale from equipment by the use of various chemical scale removal solvents which are allowed to spill onto the surface of the earth and then to seep into it. In either case, the alkaline earth metal sulfates and any other potential scale-forming materials enter the earth layers near the surface and form insoluble precipitates in these layers of the earth. Because these precipitates may also contain radium, the earth formations into which the subterranean waters enter may be subject to contamination with these radioactive scale deposits when, as is usually the case, the conditions are appropriate for precipitation of the scale. In particular, if the subterranean waters are permitted to flood onto the surface of the earth, the surface earth and closely adjacent levels of the earth may become contaminated with the radioactive deposit and after a period of time may become undesirably radioactive, with a relatively high level of background radiation.
Clearly, it is desirable to return surface formations of the earth to a condition where background radiation is reduced to a low level and also to remove insoluble materials such as alkaline earth metal scales which may have been deposited by invasion of top surface waters. The present invention provides an effective means for this to be done.